1. Field of the Invention
The present invention relates to a solid electrolytic capacitor having a smaller equivalent series resistance.
2. Description of Related Art
Solid electrolytic capacitors exhibit good high-frequency characteristics, and have a large capacity although small in size. For this reason, the solid electrolytic capacitors are widely used for power supply circuits of various electronic appliances including personal computers and imaging devices. In addition, the emergence of higher-performance mobile appliances typified by cellular phones and potable music players demands smaller-sized capacitors with a larger capacity.
FIGS. 5A to 5C show cross-sectional structural diagrams of a solid electrolytic capacitor of related art. FIG. 5A is a cross-sectional view of the conventional type of solid electrolytic capacitor 1. FIG. 5B is a cross-sectional view of solid electrolytic capacitor 1 taken along the B-B line of FIG. 5A. FIG. 5C is a cross-sectional view of solid electrolytic capacitor 1 taken along the C-C line of FIG. 5C. An anode body, a dielectric layer and a conducting polymer layer are formed around anode lead 2. FIG. 7 shows a schematic diagram of an enlarged cross-section of the anode body. Anode body 3 is formed around anode lead 2. Anode body 3 is fabricated by sintering particles of valve metals including tantalum, niobium, titanium and aluminum, into a cuboid shape.
Dielectric layer 4 is formed on each surface of anode body 3 and anode lead 2. Dielectric layer 4 is formed thereon through oxidizing each surface of anode body 3 and anode lead 2, for example, by anodic oxidation process. After an oxidant is applied to the top of dielectric layer 4, solid electrolyte layer 5 made of a conducting polymer such as polypyrrole, polyaniline or the like is formed in a way that dielectric layer 4 is covered with solid electrolyte layer 5, and in a way that spaces are embedded with solid electrolyte layer 5.
Carbon layer 6 and silver layer 7 are formed on the top surface of solid electrolyte layer 5. Plate-shaped anode terminal 1 is connected to anode lead 2, whereas plate-shaped cathode terminal 8 is connected to silver layer 7.
Outer package 9 is formed in a cuboid shape in order to accommodate anode lead 2, anode body 3, dielectric layer 4, solid electrolyte layer 5, carbon layer 6 and silver layer 7. Outer package 9 is fabricated, for example, of an epoxy resin. Anode terminal 1 and cathode terminal 8 are drawn out of outer package 9, and extend in the mutually different directions, and as well as are bent downwards. Each extremity of these terminals is arranged along the bottom surface of outer package 9, and is used to electrically connect the solid electrolytic capacitor to a mount substrate. This type of solid electrolytic capacitor is disclosed, for example, in Japanese Patent Application Laid-Open publication No. 2004-14667.
For the purpose of causing a solid electrolytic capacitor to have a larger capacity, generally adopted is a method involving increasing the surface area of a sintered body, which is going to be used as the anode body, and which is obtained by sintering metal particles made of a valve metal or its alloy around the anode lead. A method of increasing the surface area of the sintered body includes making particles of the sintered metal or alloy smaller in diameter so as to increase the bonding density among the particles. However, when particles of a metal or alloy to be used for the sintering process are made smaller in diameter, the smaller particle diameter brings about a problem of increasing the equivalent series resistance (ESR) of the solid electrolytic capacitor.